The present invention relates to an air flow measuring instrument which forms an intake system of an internal combustion engine and measures an intake air flow of the intake system, and more particularly to a thermal type air flow measuring instrument suitable for measurement of an air flow taken in an engine of an automobile.
The prior art reference most relevant to the present invention includes U.S. Pat. No. 4,974,445 (corresponding to JP-A-2-1518). An air flow meter disclosed by the JP-A-02-1518 includes a substantially L-shaped auxiliary passage formed in a main passage by cast molding. An opened outlet surface of the auxiliary passage is provided in a plane parallel to the main passage. Thus, the disclosed air flow meter has not a construction in which the auxiliary passage is inserted into the main passage but a construction in which the main passage and the auxiliary passage are cast-molded simultaneously. Accordingly, there is an inconvenience that at least two complicated casts or molds are required and hence it is difficult to reduce the fabrication cost. In connection with this, the present invention is easy to reduce the fabrication cost since a main passage can be fabricated with a simple construction independently or by itself. The U.S. Pat. No. 4,974,445 includes a disclosure concerning the possibility of insertion of an auxiliary passage into the main passage but no disclosure concerning the detailed construction thereof. Also, in this prior art, since the length of a first path of the auxiliary passage is very large as compared with that of a second path thereof, there is an inconvenience that an error of the mounting position of the auxiliary passage including a heating resistor and so forth gives a great influence on the accuracy of measurement. To the contrary, in the present invention, since the length of a first path of the auxiliary passage is not very large as compared with that of a second path thereof or is about two times as large as that of the second path, the influence of the mounting position error on the accuracy of measurement can be reduced remarkably.
Also, an air flow measuring instrument disclosed by U.S. Pat. No. 4,709,581 (corresponding to JP-4-753853) is known. However, the U.S. Pat. No. 4,709,581 has no disclosure concerning a method for mounting and fixing a main passage, an auxiliary passage and circuit portions. Further, the air flow measuring instrument disclosed by the U.S. Pat. No. 4,709,581 has a structure in which the auxiliary passage is supported in a bridge form in the main passage with opposite ends of the auxiliary passage supported by the main passage so that a member forming the auxiliary passage completely goes across the inner diameter of the main passage. That is, the instrument disclosed by the U.S. Pat. No. 4,709,581 has not a structure, intended by the present invention, in which a circuit portion and an auxiliary passage portion are configured into a unitary module so that the module standardized independently of the size of the main passage is applicable to various internal combustion engines. Furthermore, the instrument disclosed by the U.S. Pat. No. 4,709,581 has not been put into practice because the deterioration of the accuracy of measurement is feared since the auxiliary passage has the above-mentioned bridge-like structure so that the auxiliary passage is complicatedly curved or bent and because the cost becomes high since the instrument must be formed by coupling several parts. The instrument disclosed by the U.S. Pat. No. 4,709,581 is not a structure in which sufficient consideration is given to a measure to counter the change of environment due to the arrangement of the main passage at different positions of an intake system and a measure to counter the variations of mounting of the module and the main passage.